Kidney transplantation is the treatment of choice for End-Stage Renal Disease. Failure of a kidney to function after transplantation is referred to as Delayed Graft Function (DGF).DGF occurs in up to 50% of primary deceased-donor renal transplants in the US and independently predicts reduced 1- and 5-year kidney transplant survival. Prolonged cold ischemia (CI) is a significant risk factor for DGF. The average CI of donor kidneys in the US is 24 hours. DGF increases substantially when CI exceeds 30 hours. CI occurs when the organ is cooled to 4C prior to transplant, and is associated with apoptosis of renal tubular epithelial cells (RTEC). RTEC apoptosis in the donor kidney before transplant predicts the occurrence of DGF after transplant. Our published data indicates that CI of 48 hours results in increased caspase-3 protein, RTEC apoptosis and Brush Border Injury (BBI). Mammalian hibernators, such as the 13-lined ground squirrel, employ unique methods of protection that enable them to endure several days of CI during winter. We used hibernating 13-lined ground squirrels to discover novel pathways that we could study in mice. Our preliminary data indicates that hibernating 13-lined ground squirrels do not have increased caspase-3 or RTEC apoptosis despite enduring CI with a CBT of 4C for several days (far longer than that tolerable by human and mouse kidneys) (Jani 2011, in press). Protection from apoptosis in hibernating 13-lined ground squirrels is associated with upregulation of X-linked inhibitor of apoptosis protein (XIAP). This led us to measure XIAP in mice and pig kidneys subjected to CI. Our preliminary data indicates that CI of non-hibernating mouse and pig kidneys is associated with decreased XIAP, increased capsase-3 and RTEC apoptosis. A specific goal of this grant is to determine the role of XIAP in the pathophysiology of CI-induced injury in mouse kidneys. XIAP belongs to the Inhibitor of Apoptosis Protein (IAP) family, whose members bind and inhibit caspases 3, 7, and/or 9. XIAP is inhibited by a serine protease HTRA2, which in turn can be chemically inhibited by UCF-101. A specific goal of this grant is to determine the effect of CI on XIAP protein expression in mouse kidneys during CI and kidney transplantation. Specific Aim 1 investigates the effect of XIAP up-regulation (by UCF-101) and down- regulation (by HTRA2), on RTEC apoptosis. Specific Aim 2 investigates whether reducing RTEC apoptosis by manipulation of XIAP during CI before kidney transplant, improves graft function after transplant.